It dates back to the very earliest days of the model's existence. The machine was catalogued as the Model 1 and it was rated for taxation purposes in the UK at 4 RAC Horsepower.

Google can give you more detail, but it was a system that favoured, financially, long-stroke engines which is why English car and motorcycle manufacturers stayed so long with this configuration and that we are able to enjoy the wonderful 120mm stroke (and 100mm on the 500cc 16H).